docs: add macOS prerequisites and improve Node.js version guidance

[critesjosh]

Summary

• Adds macOS-specific prerequisites (Homebrew and modern Bash) to the developer getting-started guide and operator prerequisites page, including versioned copies.
• Updates the shared Node.js version snippet to clarify that the installer will attempt to upgrade Node via nvm if available, and explains what to do if nvm is not installed.

Test plan

• Verify the docs site builds without errors (yarn build in docs/)
• Check the macOS prerequisites section renders correctly on the getting started page
• Check the macOS admonition renders correctly on the operator prerequisites page
• Confirm the updated Node.js snippet reads clearly